1. Tides
Dr. Mark A. McGinley
Honors College and Department of
Biological Sciences
Texas Tech University

2. Tides
• An ocean tide refers to the cyclic rise and fall
of seawater.

3. Philippines

4. Philippines

5. Puerto Galera, Mindoro

6. Low Tide: Puerto Galera
Mindoro Island, Philippines

7. High Tide

8. Predicted Tides (Sept 6 – 12, 2012)
Puerto Galera, Philippines

9. Tide Table
Puerto Galera, Philippines
Thu 6 Sep
HW LW HW LW
01:17 08:00 14:05 18:45
1.0 m 0.2 m 0.5 m 0.3 m
Wed 12 Sep
HW LW HW
06:37 15:35 23:06
1.0 m 0.1 m 0.6 m

10. Knowledge of Tides
• Important to sailors, fishermen, etc.

11. Knowledge of Tides

12. Tides
• Tides are caused by slight variations in gravitational
attraction between the Earth, the moon and the sun in
geometric relationship with locations on the Earth's
surface.
• Tides are periodic primarily because of the cyclical
influence of the Earth's rotation.
• Tide- EoE

13. Tides
• Basically, tides are very long-period waves that move through the
oceans in response to the forces exerted by the moon and sun.
• Tides originate in the oceans and progress toward the coastlines
where they appear as the regular rise and fall of the sea surface.
• When the highest part, or crest of the wave reaches a particular
location, high tide occurs; low tide corresponds to the lowest part
of the wave, or its trough. The difference in height between the
high tide and the low tide is called the tidal range.
• http://oceanservice.noaa.gov/education/tutorial_tides/tides01_intr
o.html
NOAA

14. Tides
• Gravity is one major force that creates tides.
In 1687, Sir Isaac Newton explained that
ocean tides result from the gravitational
attraction of the sun and moon on the oceans
of the earth

15. Brief Review of Gravity
• Newton’s law of universal gravitation states
that the gravitational attraction between two
bodies is directly proportional to their
masses, and inversely proportional to the
square of the distance between the bodies

16. Brief Review of Gravity
• F = G(mass1*mass2)/D2
• F - gravitational force between 2 bodies
• G – gravitational constant
• mass1 – mass of the first object
• mass2– mass of the second object
• D – distance between the two objects

17. Causes of Tides
• The Sun is 27 million times larger than the
Moon
• The Sun is 390 times farther from the Earth
than the Moon
• Thus, the Moon has twice the tide generating
force as the Sun

19. Causes of Tides
• The gravitational attraction between the Earth and the
moon is strongest on the side of the Earth that
happens to be facing the moon, simply because it is
closer.
• This attraction causes the water on this “near side” of
Earth to be pulled toward the moon.
• As gravitational force acts to draw the water closer to
the moon, inertia attempts to keep the water in place.
– But the gravitational force exceeds it and the water is
pulled toward the moon, causing a “bulge” of water on the
near side toward the moon

20. Causes of Tides

21. Causes of Tides
• The sun also plays a major role, affecting the
size and position of the two tidal bulges. The
interaction of the forces generated by the
moon and the sun can be quite complex.
• Over time, the positions of the Sun and Moon
change relative to the Earth’s equator. The
changes in their relative positions have a
direct effect on daily tidal heights and tidal
current intensity.

22. Causes of Tides
• As the moon revolves around the Earth, its
angle increases and decreases in relation to
the equator.
– This is known as its declination.
• The two tidal bulges track the changes in lunar
declination, also increasing or decreasing their
angles to the equator.
• http://oceanservice.noaa.gov/education/tutor
ial_tides/media/tide04_400.gif

23. Causes of Tides
• Similarly, the sun’s relative position to the equator
changes over the course of a year as the Earth rotates
around it. The sun’s declination affects the seasons as
well as the tides.
• During the vernal and autumnal equinoxes—March 21
and September 23, respectively—the sun is at its
minimum declination because it is positioned directly
above the equator.
• On June 21 and December 22—the summer and winter
solstices, respectively—the sun is at its maximum
declination, i.e., its largest angle to the equator

24. Causes of Tides
• Most coastal areas, with some exceptions, experience
two high tides and two low tides every lunar day
• Similarly, a lunar day is the time it takes for a specific
site on the Earth to rotate from an exact point under
the moon to the same point under the moon.
– Unlike a solar day, however, a lunar day is 24 hours and 50
minutes.
– The lunar day is 50 minutes longer than a solar day
because the moon revolves around the Earth in the same
direction that the Earth rotates around its axis. So, it takes
the Earth an extra 50 minutes to “catch up” to the moon

25. Causes of Tides
• http://oceanservice.noaa.gov/education/tutorial
_tides/media/tide05a_300.gif
• Since the Earth rotates through two tidal “bulges”
every lunar day, we experience two high and two
low tides every 24 hours and 50 minutes.
Here, we see the relationship between the tidal
cycle and the lunar day. High tides occur 12 hours
and 25 minutes apart, taking six hours and 12.5
minutes for the water at the shore to go from
high to low, and then from low to high.

26. Causes of Tides
• The moon is a major influence on the Earth’s
tides, but the sun also generates considerable
tidal forces.
• Solar tides are about half as large as lunar
tides and are expressed as a variation of lunar
tidal patterns, not as a separate set of tides

27. Cause of Tides

28. Causes of Tides
• When the sun, moon, and Earth are in alignment
(at the time of the new or full moon), the solar
tide has an additive effect on the lunar
tide, creating extra-high high tides, and very
low, low tides—both commonly called spring
tides.
• One week later, when the sun and moon are at
right angles to each other, the solar tide partially
cancels out the lunar tide and produces moderate
tides known as neap tides.
• During each lunar month, two sets of spring tides
and two sets of neap tides occur

29. Causes of Tides
• http://oceanservice.noaa.gov/education/tutor
ial_tides/media/tide06a_450.gif

30. Causes of Tides
• Just as the angles of the sun, moon and Earth
affect tidal heights over the course of a lunar
month, so do their distances to one another.
• Because the moon follows an elliptical path
around the Earth, the distance between them
varies by about 31,000 miles over the course
of a month

31. Causes of Tides

32. Causes of Tides
• . Once a month, when the moon is closest to
the Earth (at perigee), tide-generating forces
are higher than usual, producing above-
average ranges in the tides.
• About two weeks later, when the moon is
farthest from the Earth (at apogee), the lunar
tide-raising force is smaller, and the tidal
ranges are less than average

33. Causes of Tides
• A similar situation occurs between the Earth
and the sun. When the Earth is closest to the
sun (perihelion), which occurs about January 2
of each calendar year, the tidal ranges are
enhanced. When the Earth is furthest from
the sun (aphelion), around July 2, the tidal
ranges are reduced